Evolution of structural distortion in BiFeO₃ thin films probed by second-harmonic generation

Area of Science:

• Materials Science
• Condensed Matter Physics
• Thin Film Technology

Background:

• Bismuth Ferrite (BiFeO3) thin films are crucial for magnetoelectric devices.
• Understanding the structural and symmetry evolution with thickness is vital but remains debated.

Purpose of the Study:

• To investigate the phase-related symmetry evolution of BiFeO3 thin films as a function of thickness.
• To clarify the controversial structural changes in BiFeO3 films with varying thickness.

Main Methods:

• Optical second-harmonic generation (SHG) technique for symmetry analysis.
• High-resolution X-ray diffractometry reciprocal space mapping for crystalline structure.
• Piezoresponse force microscopy (PFM) for local piezoelectric response.

Main Results:

• BiFeO3 thin films below 60 nm exhibit 4mm point group symmetry.
• Disappearance of fourfold rotational symmetry in SHG s-out patterns indicates the R-phase.
• The tensor element ratio χ31/χ15 approaches 1 as film thickness decreases, signifying increased symmetry.

Conclusions:

• Film thickness significantly influences the symmetry and phase of BiFeO3.
• A transition to the R-phase occurs with decreasing thickness, accompanied by an increase in symmetry.